1.4 Sea-Floor Spreading

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Vocabulary:

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subduction-process by which the ocean floor sinks beneath a deep-ocean trench and back into the mantle
mid-ocean ridge-longest chain of mountains in the world
sonar-device that bounces sound waves off underwater objects and then records the echoes of these sound waves
sea-floor spreading-process that continually adds new material to the ocean floor
deep-ocean trenches-deep underwater canyons

Outline:

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Summary

• There are many bizzare animals deep under the water

Mapping the Mid-Ocean Ridge

• curves like seam of baseball
• under hundred of meters of water
• more than 50,000 kilometers long

Evidence from Sea-Floor Spreading

• At the mid-ocean ridge, molten material rises from the mantle and erupts.
• The molten material then spreads out pushing older rock to both sides of the ridge
• Evidence from Molten Material
  • found strange rocks shaped like:
    • pillows
    • toothpaste
  • such rocks can form only when molten material hardens quickly after erupting under water
• Evidence from Magnetic Stripes
  • when cooled, iron bits inside lined up in direction of Earth's magnetic poles
  • rocks hardened at the same time has the same magnetic memory
  • when rocks hardened completely, it locked the iron bits in place giving the rocks a permanent "magnetic memory"
• Evidence from Drilling Samples
  • found-the farther from ridge the sample was taken, the older the rocks were-shows sea-floor spreading took place

Subduction at Deep-Ocean Trenches

• convection currents under the lithosphere pushes new crust that forms at the mid-ocean ridge away from the ridge and toward a deep ocean trench
• at deep-ocean trenches, subduction allows part of the ocean floor to sink back into the mantle for over tens of million years

Subduction and Earth's Oceans

• Sea-floor spreading can change the size and shape of the ocean
• Subduction in the Pacific Ocean
  • the Pacific Ocean is shrinking because sometimes a deep-ocean trench swallows more oceanic crust than the mid-ocean ridge can make
• Subduction in the Atlantic Ocean
  • the Atlantic Ocean is expanding because it only has a few short trenches so the ocean floor has nowhere to go